The present invention relates to a device for spraying flux to apply it to the rear of a printed circuit board which is loaded with electronic parts on the front thereof.
Many of modern electronic equipment accommodate therein one or more printed circuit boards each being loaded with necessary electronic parts on the front thereof. After electronic parts have been mounted on the front of such a circuit board, flux is applied to the rear of the circuit board while the circuit board is transported by a conveyor or similar transport mechanism. For the application of flux, various types of applying devices are extensively used such as a foam type applying device and a spray type applying device. A spray type flux applying device, for example, has at least one nozzle to which flux and air are fed at the same time. Having a nozzle hole at the tip thereof, the nozzle atomizes the flux and sprays it onto a printed circuit board though the nozzle hole.
It is a common practice with the conventional spray type flux applying device to affix one or more nozzles to the casing of the device. Usually, the flux is deposited in an oval configuration when sprayed onto an object. Assume that a single nozzle is affixed to the casing of the device to spray flux onto the rear of a printed circuit board which is transported by, for example, a conveyor. Then, the effective application area as measured on the rear of the circuit board is about 150 millimeters at most in terms of the longer diameter of the oval and is generally smaller than the size of an ordinary printed circuit board. Hence, it has been customary to mount a plurality of nozzles rigidly on the casing of the applying device. However, it is extremely difficult to apply flux uniformly over the predetermined application area of a printed circuit board by use of such a spray type applying device due to three major problems as follows. A first problem is that each nozzle cannot uniformly apply flux to the entire effective application area thereof. A second problem is that the sprays from nearby nozzles interfere with each other in a portion where the effective application areas of the nozzles overlap. This is partly because the amount of flux to be sprayed, drop size and effective application area slightly differ from one nozzle to another due to the limited machining accuracy and partly because a plurality of such nozzles are used. A third problem is that not all of the printed circuit boards have identical dimensions.
Under the above circumstances, it is necessary to adjust, mainly by hand, the position and angle of each nozzle, the positional relation of the nozzles to one another, and the number of nozzles so as to adjust the application area, nozzle by nozzle. This is extremely time- and labor-consuming and requires an expert. In practice, therefore, the adjustment of the application area is rarely performed. Usually, an application area matching a printed circuit board having the maximum size is fixedly set and applied even to printed circuit boards of smaller sizes. As a result, sprayed flux deposits not only on the circuit boards but also on the structural members of the applying device and the conveyor, resulting in the waste of flux. Especially, a collecting device for collecting the drops of flux not deposited on the circuit boards and conveyor is heavily smeared and has to be cleaned or replaced by troublesome operations.
The above-described type of flux applying device has a non-hermetic reservoir storing flux therein, and a conduit communicating the reservoir to the nozzle. The feed of the flux from the reservoir to the nozzle is implemented by the difference in level between the nozzle and the liquid level in the reservoir, i.e., the weight of the flux. This brings about a problem that as the liquid level in the reservoir changes due to the consumption of the flux or the supply of fresh flux to the reservoir, the pressure acting on the flux inside the reservoir and, therefore, the amount of flux to be sprayed from the nozzle changes. In addition, since the reservoir is not hermetically closed, the solvent contained in the flux is apt to evaporate with the result that the concentration of the flux changes over time.
While the conventional device feeds flux and air to the nozzle, it cannot control the feed of flux and that of air independently of each other, i.e., it simply starts and stops feeding flux and air at the same time. It follows that when the spray of flux from the nozzle is stopped, the flux is left at the tip of the nozzle and then dried to clog the nozzle hole, obstructing stable spraying. More specifically, comparatively large drops of flux are ejected from the nozzle at the beginning of spraying, preventing the flux from being applied in a uniform distribution to an object.